1. Field of the Invention
The present invention relates to a gas mixture method and apparatus for generating ion beam, and more particularly to a method and apparatus of prolonging lifetime of an ion source for generating an ion beam.
2. Description of the Prior Art
Electronic products bring convenience to humans' life and all these products rely on controller chips for operation. Semiconductors have desired electrical properties and thus are widely used for manufacturing chips. Generally, semiconductor wafers need to be doped with chemical elements to reach different electrical properties for different uses. For example, n-type semiconductors can be doped with electron donors (e.g. Group V elements) to increase concentration of free electrons and it affords rise in conductivity.
Currently, ion implanting process is mainly adopted by industry for doping because it can deposit ions onto wafer surface homogeneously. An ion implanter system comprises gas source, a reaction chamber, magnets, and extraction electrodes. While the dopant gas is injected into the chamber, electrons emitted from the hot cathode collide with gas molecules to generate ions. Then magnets establish a magnet field to confine those electrons to prevent them from striking the reaction chamber before colliding with gas molecules, thus to increase electron density and the plasma density. Extraction electrode placed adjacent to the aperture of the reaction chamber extracts the ions out of the plasma to form an ion beam outside the chamber along a desired trajectory. Species of dopant gas contribute to electrical presence of doped wafers such as n-type semiconductors or p-type semiconductors and may comprise AsH3, GeF4, BF3, CO2, PH3 etc. The reaction chamber which accommodates the anode, cathode, reactant gases is called ion source. Stable and uniform ion beam generated from the ion source is required. However, quality of the ion beam may degrade after operating in a period of time; the interval between operation failures is called mean time between failures (MTBF) or lifetime and is considered a critical factor regarding doping process.
Operation failure may result from several reasons. First, there are probably impure particles accumulated on the cathode so to effect thermal emission of electron flow density, which can render reduced density or stability of ion beam. The situation especially occurs when fluorine is contained in dopant gas (e.g. GeF4, BF3). As we know, fluorine ion has strong oxidation ability and can oxidize metal atoms or other molecules. After undergoing doping process, excessive fluorine continue to exist in the reaction chamber and may erodes chamber wall. Materials of the chamber such as metal ions are then released and sputtered on the cathode under reacting atmosphere. Thus, thermal emission of electrons from the cathode is impeded, which results in unstable ion beam.
Besides, since plasma (ionized gas) is formed in the chamber, positive ions also can possibly accelerate to bombard the cathode to dissociate metal materials of the cathode, such as W or Mo, etch away the cathode material to shorten cathode lifetime; and dissociated materials may be attached to the chamber wall. However, if we add some fluorine containing gas, such as CF4, the halogen effect caused by fluorine molecules can move W or Mo materials from the cooler chamber wall back to the cathode surface to prolong cathode's lifetime. The competing processes of oxidation on W or Mo cathode surfaces versus W or Mo re-deposition on W or Mo cathode surface make the ratio between dopant gas and fluorine containing minor gas critical for the best ion source performance.
Other factors degenerating quality of ion beam relate to the aforementioned phenomenon. While the cathode or chamber wall are etched or attached with impure particles thereon, which means inner surface of the chamber becomes rough, electric field may be disturbed. For example, a tiny protrusion may discharge or arc under the electric field. Consequently, plasma distribution may shift or be distorted to cause undesired results for generating the ion beam.
The above-mentioned problems cause damages to the ion source, which not only affect quality of the ion beam, but also decrease lifetime of the ion source. To solve these problems, regular maintenance such as cleaning or polishing is necessary to prolong lifetime of the ion source. Unfortunately, these actions are time-wasted and complicated steps because it requires entire dissemblance of the ion source head; maintenance cannot be performed frequently in practice otherwise the productivity of an ion implanter would be greatly impacted. Therefore, to enhance ion implanter productivity and reduce maintenance cost through prolonging lifetime of ion source, an alternative step of simple maintenance allows in-situ cleaning during generating ion beam needs to be developed.